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Old 29th January 2007, 06:20 AM   #1
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Default Help - CFB in a KT88 SE amp

I would really appreciate some guidance!

I've been designing my own single ended amp using a 6SL7, KT88 in UL and a Hammond 1627 output transformer. The amp performs OK on the test bench putting out about 10 watts before visable clipping on the scope. The KT88 uses cathode bias.

Recently I tried to add CFB by putting the transformer secondary (8 ohms) in the KT88 cathode circuit (with the secondary winding at the earthy end of the circuit). I find that the amplifier gain increases by about 50%, the frequency resonse shrinks and the square wave looks like there is RF present. Have I got the secondary winding in the wrong way around? I cannot find much on the www about the problem.

Any helpful comments for a newbie?

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Old 29th January 2007, 06:35 AM   #2
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Actually I think I just answered my own question! I'll swap the secondary around tomorrow and re-test

It's getting late - at least that's my excuse

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Old 29th January 2007, 08:39 AM   #3
Fuling is offline Fuling  Sweden
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I think Tubelab has mentioned that the phase must be inverted to get CFB from Hammond OPTīs.
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Old 29th January 2007, 01:54 PM   #4
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Yes, the Hammond 162X series transformers are among those that must be wired up "backwards". The schematic of my amp (including the feedback switch and a 1628SEA transformer) is here:
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 30th January 2007, 04:36 AM   #5
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Thanks guys,

I reversed the secondary and now all is as expected! I'm surprised at how effect the CFB is and how good the response of the Hammond 1627 is.

With only CFB (ie. no global feedback) the response is -3dB 20hz to 29.4KHz, -1db 40Hz to 20KHz. Overall gain is 13.8x. There is a 20mV 50Hz hum on the amp output so I need to experiment with grounding one of the heater wires.

Thanks for your reply!
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